1. Field of the Invention
The present invention relates to a thin-film resistor, and more particularly to a thin-film resistor employed in a semiconductor wafer and its method of formation.
2. Description of the Prior Art
A thin-film resistor is not affected by temperature fluctuations and has low conductivity and stable resistance. Therefore, it is often used in the semiconductor circuit design. However, if the resistance layer of the thin-film resistor is uneven, the resistance will become unstable.
Please refer to FIG. 1. FIG. 1 is a sectional schematic diagram of the thin-film resistor 20 employed in the semiconductor wafer 10 according to the prior art. A thin-film resistor 20 is positioned on a semiconductor wafer 10 and comprises a first dielectric layer 12, two conductive layers 14, a second dielectric layer 16, and a resistance layer 18. The first dielectric layer 12 is positioned on the surface of the semiconductor wafer 10. The two conductive layers 14 are positioned in a predetermined area of the first dielectric layer 12. The second dielectric layer 16 positioned on the two conductive layers 14. There are two separate openings in the second dielectric layer 16 above the two conductive layers 14. The resistance layer 18 is positioned in a predetermined area of the second dielectric layer 16 and fills the two openings above the two conductive layers 14. Two ends of the two conductive layers 14 are in separate contact with the resistance layer 18, so the two conductive layers 14 are used as two electrical terminals of the resistance layer 18 when the semiconductor wafer 10 is electrically connected to external components.
The two conductive layers 14 are positioned in the predetermined area of the first dielectric layer 12 as the first step in the formation of the thin-film resistor 20. In the process, the surface of the semiconductor wafer 10 becomes uneven. The second dielectric layer 16 and the resistance layer 18 then sequentially deposited on the semiconductor wafer 10, but a step coverage problem occurs. The thickness of the deposited resistance layer 18 is uneven so as to cause adverse effects. The two conductive layers 14 are electrically linked with the resistance layer 18 such that greater resistance is generated in the thinner region of the resistance layer 18 and smaller resistance is generated in the thicker region of the resistance layer 18. Thus, the resistance generated from the thin-film resistor 20 varies based on the thickness of the resistance layer 18.